Dissolved oxygen deficits in a shallow eutrophic aquatic ecosystem (fishpond) - Sediment oxygen demand and water column respiration alternately drive the oxygen regime

Biological processes tend to dominate the oxygen regime of productive waters. However, in shallow aquatic ecosystems, it is unclear whether the oxygen regime is driven by oxygen production and consumption in the water column or by sediment oxygen demand (SOD). In managed eutrophic ecosystems, this question is especially important in the context of extreme daily oscillations of dissolved oxygen (DO) that could breach physiological limits of heterotrophic aerobic organisms. High-frequency measurement of DO, temperature, global radiation (Gl.Rad.), and pH in a 0.6 m deep, 22 ha eutrophic fishpond Rod (Czech Republic) shows that the oxygen regime depended on the ecosystem state. Over the clearwater period in the early season, the DO level reflected ecosystem heterotrophy with relatively low daily DO oscillations. However, during the summer phytoplankton bloom, the fishpond was primarily autotrophic with extreme DO fluctuation. During late summer, a collapse of the phytoplankton bloom and an associated shift towards heterotrophy and DO deficit frequently occur. In-situ mesocosm experiments in Rod fishpond were conducted throughout 2018 and 2019 growing seasons, to address the importance of SOD to the oxygen regime. We enclosed the water column in transparent and opaque/dark plastic cylinders open or closed to the sediment. The results show that the proportional contribution of SOD to total respiration decreased from 70 to 90% at low phytoplankton biomass (expressed as Chlorophyll-a (Chl-a) concentration) to approximately 10% at phytoplankton bloom. At night, the difference between the oxygen consumption in the cylinders with or without sediment was statistically significant, when the concentration of Chl-a was <100 mu g.L-1 On the contrary, the difference was not significant when the concentration of Chl-a was >100 mu g.L-1 This revealed that the impact of SOD is negligible at high phytoplankton biomass. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The research indicates that the oxygen regime in aquatic ecosystems varies significantly depending on the ecosystem state. During the clearwater period, the fishpond is mainly heterotrophic with low oxygen fluctuations, while in the summer phytoplankton bloom period, the fishpond shifts to autotrophy with extreme oxygen fluctuations.